Novel phosphor systems for a white light emitting diode (LED)

ABSTRACT

Novel phosphor systems for a white LED are disclosed. The phosphor systems are excited by a non-visible to near-UV radiation source having an excitation wavelength ranging from about 250 to 420 nm. The phosphor system may comprise one phosphor, two phosphors, and may include optionally a third and even a fourth phosphor. In one embodiment of the present invention, the phosphor is a two phosphor system having a blue phosphor and a yellow phosphor, wherein the long wavelength end of the blue phosphor is substantially the same wavelength as the short wavelength end of the yellow phosphor. Alternatively, there may be a wavelength gap between the yellow and blue phosphors. The yellow phosphor may be phosphate or silicate-based, and the blue phosphor may be silicate or aluminate-based. Single phosphor systems excited by non-visible radiation are also disclosed. In other embodiments of present invention, a single phosphor is used to produce white light illumination, the single phosphor having a broad emission spectrum with a peak intensity ranging from about 520 to 560 nm.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the present invention are directed in general to novelphosphors for use in a white light illumination system such as a whitelight emitting diodes (LED). In particular, the white LED's of thepresent invention comprise a radiation source emitting in thenon-visible to near ultraviolet (UV) to purple wavelength range, a firstluminescent material comprising a blue phosphor, and a secondluminescent material comprising a yellow phosphor.

2. State of the Art

It has been suggested that white light illumination sources based whollyor in part on the light emitting diode will likely replace theconventional, incandescent light bulb. Such devices are often referredto as “white LED's,” although this may be somewhat of a misnomer, as anLED is generally the component of the system that provides the energy toanother component, a phosphor, which emits light of more-or-less onecolor; the light from several of these phosphors, possibly in additionto the light from the initial pumping LED are mixed to make the whitelight.

Nonetheless, white LED's are known in the art, and they are relativelyrecent innovations. It was not until LED's emitting in theblue/ultraviolet region of the electromagnetic spectrum were developedthat it became possible to fabricate a white light illumination sourcesbased on an LED. Economically, white LED's have the potential to replaceincandescent light sources (light bulbs), particularly as productioncosts fall and the technology develops further. In particular, thepotential of a white light LED is believed to be superior to that of anincandescent bulbs in lifetime, robustness, and efficiency. For example,white light illumination sources based on LED's are expected to meetindustry standards for operation lifetimes of 100,000 hours, andefficiencies of 80 to 90 percent. High brightness LED's have alreadymade a substantial impact on such areas of society as traffic lightsignals, replacing incandescent bulbs, and so it is not surprising thatthey will soon provide generalized lighting requirements in homes andbusinesses, as well as other everyday applications.

Chromaticity Coordinates on a CIE Diagram, and the CRI

White light illumination is constructed by mixing various or severalmonochromatic colors from the visible portion of the electromagneticspectrum, the visible portion of the spectrum comprising roughly 400 to700 nm. The human eye is most sensitive to a region between about 475and 650 nm. To create white light from either a system of LED's, or asystem of phosphors pumped by a short wavelength LED, it is necessary tomix light from at least two complementary sources in the properintensity ratio. The results of the color mixing are commonly displayedin a CIE “chromaticity diagram,” where monochromatic colors are locatedon the periphery of the diagram, and white at the center. Thus, theobjective is to blend colors such that the resulting light may be mappedto coordinates at the center of the diagram.

Another term of art is “color temperature,” which is used to describethe spectral properties of white light illumination. The term does nothave any physical meaning for “white light” LED's, but it is used in theart to relate the color coordinates of the white light to the colorcoordinates achieved by a black-body source. High color temperatureLED's versus low color temperature LED's are shown at www.korry.com.

Chromaticity (color coordinates on a CIE chromaticity diagram) has beendescribed by Srivastava et al. in U.S. Pat. No. 6,621,211. Thechromaticity of the prior art blue LED-YAG:Ce phosphor white lightillumination system described above are located adjacent to theso-called “black body locus,” or BBL, between the temperatures of 6000and 8000 K. White light illumination systems that display chromaticitycoordinates adjacent to the BBL obey Planck's equation (described atcolumn 1, lines 60-65 of that patent), and are desirable because suchsystems yield white light which is pleasing to a human observer.

The color rendering index (CRI) is a relative measurement of how anillumination system compares to that of a black body radiator. The CRIis equal to 100 if the color coordinates of a set of test colors beingilluminated by the white light illumination system are the same as thecoordinates generated by the same set of test colors being irradiated bya black body radiator.

Prior Art Approaches to Fabricating White LED's

In general, there have been three general approaches to making whiteLED's. One is to combine the output from two or more LED semiconductorjunctions, such as that emitted from a blue and a yellow LED, or morecommonly from a red, green, and blue (RGB) LED's. The second approach iscalled phosphor conversion, wherein a blue emitting LED semiconductorjunction is combined with a phosphor. In the latter situation, some ofthe photons are down-converted by the phosphor to produce a broademission centered on a yellow frequency; the yellow color then mixeswith other blue photons from the blue emitting LED to create the whitelight.

Phosphors are widely known, and may be found in such diverseapplications as CRT displays, UV lamps, and flat panel displays.Phosphors function by absorbing energy of some form (which may be in theform of a beam of electrons or photons, or electrical current), and thenemitting the energy as light in a longer wavelength region in a processknown as luminescence. To achieve the required amount of luminescence(brightness) emitted from a white LED, a high intensity semiconductorjunction is needed to sufficiently excite the phosphor such that itemits the desired color that will be mixed with other emitted colors toform a light beam that is preceived as white light by the human eye.

In many areas of technology, phosphors are zinc sulfides or yttriumoxides doped with transition metals such as Ag, Mn, Zn, or rare earthmetals such as Ce, Eu, or Tb. The transition metals and/or rare earthelement dopants in the crystal function as point defects, providingintermediate energy states in the material's bandgap for electrons tooccupy as they transit to and from states in the valence band orconduction band. The mechanism for this type of luminescence is relatedto a temperature dependent fluctuation of the atoms in the crystallattice, where oscillations of the lattice (phonons) cause displacedelectron to escape from the potential traps created by theimperfections. As they relax to initial state energy states they mayemit light in the process.

U.S. Pat. No. 5,998,925 to Shimizu et al. discloses the use of a 450 nmblue LED to excite a yellow phosphor comprising ayttrium-aluminum-garnet (YAG) fluorescent material. In this approach aInGaN chip functions as a visible, blue-light emitting LED, and a ceriumdoped yttrium aluminum garnet (referred to as “YAG:Ce”) serves as asingle phosphor in the system. The phosphor typically has the followingstoichiometric formula: Y₃Al₅O₁₂:Ce³⁺. The blue light emitted by theblue LED excites the phosphor, causing it to emit yellow light, but notall the blue light emitted by the blue LED is absorbed by the phosphor;a portion is transmitted through the phosphor, which then mixes with theyellow light emitted by the phosphor to provide radiation that isperceived by the viewer as white light.

U.S. Pat. No. 6,504,179 to Ellens et al. disclose a white LED based onmixing blue-yellow-green (BYG) colors. The yellow emitting phosphor is aCe-activated garnet of the rare earths Y, Tb, Gd, Lu, and/or La, where acombination of Y and Tb was preferred. In one embodiment the yellowphosphor was a terbium-aluminum garnet (TbAG) doped with cerium(Tb₃Al₅O₁₂—Ce). The green emitting phosphor comprised a CaMgchlorosilicate framework doped with Eu (CSEu), and possibly includingquantities of further dopants such as Mn. Alternative green phosphorswere SrAl₂O₄:Eu²⁺ and Sr₄Al₁₄O₂₅:Eu²⁺. New material in replace 5998925,using 450 m Blue LED to excite mixture of green and yellow phosphors(Tb₃Al₅O₁₂—Ce).

U.S. Pat. No. 6,649,946 to Bogner et al. disclosed yellow to redphosphors based on alkaline earth silicon nitride materials as hostlattices, where the phosphors may be excited by a blue LED emitting at450 nm. The red to yellow emitting phosphors uses a host lattice of thenitridosilicate type M_(x)Si_(y)N_(z):Eu, wherein M is at least one ofan alkaline earth metal chosen from the group Ca, Sr, and Ba, andwherein z=⅔x+ 4/3y. One example of a material composition isSr₂Si₅N₈:Eu²⁺. The use of such red to yellow phosphors was disclosedwith a blue light emitting primary source together with one or more redand green phosphors. The objective of such a material was to improve thered color rendition R9 (adjust the color rendering to red-shift), aswell as providing a light source with an improved overall colorrendition Ra.

U.S. patent application 2003/0006702 to Mueller-Mach disclosed a lightemitting device having a (supplemental) fluorescent material thatreceives primary light from a blue LED having a peak wavelength of 470nm, the supplemental fluorescent material radiating light in the redspectral region of the visible light spectrum. The supplementaryfluorescent material is used in conjunction with a main fluorescentmaterial to increase the red color component of the composite outputlight, thus improving the white output light color rendering. In a firstembodiment, the main fluorescent material is a Ce activated and Gd dopedyttrium aluminum garnet (YAG), while the supplementary fluorescentmaterial is produced by doping the YAG main fluorescent material withPr. In a second embodiment, the supplementary fluorescent material is aEu activated SrS phosphor. The red phosphor may be, for example,(SrBaCa)₂Si₅N₈: Eu²⁺. The main fluorescent material (YAG phosphor) hasthe property of emitting yellow light in response to the primary lightfrom the blue LED. The supplementary fluorescent material adds red lightto the blue light from the blue LED and the yellow light from the mainfluorescent material.

Disadvantages of the Prior Art Blue LED-YAG:Ce Phosphor White LightIllumination System

The blue LED-YAG:Ce phosphor white light illumination system of theprior art has disadvantages. One disadvantage is that this illuminationsystem produces white light with color temperatures ranging from 6000 to8000 K, which is comparable to sunlight, and a typical color renderingindex (CRI) of about 70 to 75. These specifications are viewed as adisadvantage because in some instances white light illumination systemswith a lower color temperature are preferred, such as between about 3000and 4100 K, and in other cases a higher CRI is desired, such as above90. Although the color temperature of this type of prior art system canbe reduced by increasing the thickness of the phosphor, the overallefficiency of the system decreases with such an approach.

Another disadvantage of the blue LED-YAG:Ce phosphor white lightillumination system of the prior art is that the output of the systemmay vary due to manufacturing inconsistencies of the LED. The LED coloroutput (quantified by the spectral power distribution and the peakemission wavelength) varies with the bandgap of the LED active layer andwith the power that is applied to the LED. During production of theLEDs, a certain percentage are manufactured with active layers whoseactual bandgaps are either larger or smaller than the desired width.Thus, the color output of such LEDs deviates from desired parameters.Furthermore, even if the bandgap of a particular LED has the desiredwidth, during operation of the white light illumination systemfrequently deviates from the desired value. This also causes the coloroutput of the LED to deviate from desired parameters. Since the whitelight emitted from the illumination system contains a blue componentfrom the LED, the characteristics of the light output from theillumination system may vary as the characteristics of the light outputfrom the LED vary. A significant deviation from desired parameters maycause the illumination system to appear non-white; i.e., bluish if theLED output is more intense than desired, and yellowish if less intense.

LEDs that emit in the visible, such as the prior art LEDs having anInGaN active layer, suffer from the disadvantage that a variation in theIn to Ga ratio during the deposition of the InGaN layer results in anactive layer whose band gap width which may deviate from the desiredthickness. Variations in the color output of the phosphor (theluminescent portion of the white light illumination system) do notdepend as much on compositional variations of the phosphor as they do oncompositional variations in the blue LED. Futhermore, the manufacture ofthe phosphor is less prone to compositional errors than is themanufacture of the LED. Another advantage of using excitationwavelengths around 400 nm from the radiation source is that an LED suchas GaInAlN has its highest output intensity around this range.

Thus, what is needed in the art is a white light illumination systemwith a radiation source emitting substantially in the non-visible,phosphors whose color output is stable, and whose color mixing resultsin the desired color temperature and color rendering index.

SUMMARY OF THE INVENTION

In one embodiment of the present invention, a white LED comprises aradiation source configured to emit radiation having a wavelengthranging from about 250 to 420 nm; a yellow phosphor configured to absorbat least a portion of the radiation from the radiation source and emitlight with peak intensity in a wavelength ranging from about 530 to 590nm; and a blue phosphor configured to absorb at least a portion of theradiation from the radiation source and emit light with peak intensityin a wavelength ranging from about 470 to 530 nm.

The 250 to 400 nm portion of the radiation emitted by the radiationsource is substantially non-visible ultraviolet (UV) radiation, and the400 to 420 nm portion is substantially near-UV light from theelectromagnetic spectrum. The radiation source may be a light emittingdiode (LED).

According to this embodiment, the yellow phosphor may be either asilicate-based phosphor or a phosphate-based phosphor. If the yellowphosphor is a silicate-based phosphor, it may have the formulaA₂SiO₄:Eu²⁺, where A is at least one divalent metal selected from thegroup of consisting of Sr, Ca, Ba, Mg, Zn, and Cd. There may be morethan one type of the divalent metal A present in any one phosphor. Thepurpose of the Eu is to serve as the luminescent activator, substitutingfor at least a portion of any of the A divalent metals, wherein the Euis present in about a zero to 10% mole ratio. An example of a yellowphosphor fitting this description isSr_(0.98-x-y)Ba_(x)Ca_(y)Eu_(0.02)SiO₄, where 0≦x≦0.8 and 0≦y≦0.8.Alternatively, the yellow phosphor may be phosphate-based phosphoraccording to the formula (Sr_(1-x-y)Eu_(x)Mn_(y))₂P_(2+z)O₇, where0.03≦x≦0.08, 0.06≦y≦0.16, and 0<z≦0.05.

Further according to this embodiment, the blue phosphor may be asilicate-based phosphor or an aluminate-based phosphor. For example, theblue phosphor may fit the descriptionSr_(0.98-x-y)Mg_(x)Ba_(y)Eu_(0.02)SiO₄; where 0≦x≦1.0; and 0≦y≦1.0. Ifthe blue phosphor is an aluminate, it may either have the formulaSr_(1-x)MgEu_(x)Al₁₀O₁₇; where 0.2<x≦1.0, or Sr_(x)Eu_(0.1)Al₁₄O₂₅,wherein x is less than 4. In one embodiment, the formula for thisphosphor is Sr_(3.9)Eu_(0.1)Al₁₄O₂₅.

In an alternative embodiment, the white LED comprises a radiation sourceconfigured to emit radiation having a wavelength ranging from about 250to 420 nm; a yellow phosphor configured to absorb at least a portion ofthe radiation from the radiation source and emit light with a peakintensity in a wavelength ranging from about 550 to 590 nm; and a bluephosphor configured to absorb at least a portion of the radiation fromthe radiation source and emit light with a peak intensity in awavelength ranging from about 480 to 510 nm.

In other embodiments, the yellow phosphor is configured to absorb atleast a portion of the radiation from the radiation source and emitlight in a wavelength ranging from about 550 to 575 nm; and the bluephosphor configured to absorb at least a portion of the radiation fromthe radiation source and emit light in a wavelength ranging from about480 to 495 nm.

An alternative embodiment of the present invention is a single phosphorsystem for a white LED, the phosphor system comprising a radiationsource configured to emit radiation having a wavelength ranging fromabout 250 to 420 nm, the single phosphor configured to absorb at least aportion of the radiation from the radiation source and emit a broadspectrum light with a peak intensity in a wavelength ranging from about520 to 560 nm.

In each of these cases, the yellow phosphor may be silicate orphosphate-based, and the blue phosphor may be silicate oraluminate-based, as discussed elsewhere in this disclosure.

Further embodiments of the present invention include methods ofproducing white light illumination from a one or two-phosphor system(the phosphor system optionally including even a third and/or fourthphosphor). Such methods may comprise the steps of providing a radiationsource configured to emit radiation having a wavelength ranging fromabout 250 to 420 nm; exposing a yellow phosphor to at least a portion ofthe radiation from the radiation source to produce light having awavelength ranging from about 530 to 590 nm; exposing a blue phosphor toat least a portion of the radiation from the radiation source to producelight having a wavelength ranging from about 470 to 530 nm; and mixingthe light from the yellow phosphor with the light from the blue phosphorto produce the white illumination.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a prior art illumination systemcomprising a radiation source that emits in the visible and a phosphorthat emits in response to the excitation from the radiation source,wherein the light produced from the system is a mixture of the lightfrom the phosphor and the light from the radiation source;

FIG. 2 is a schematic representation of a prior art illumination systemcomprising a radiation source that emits in the non-visible such thatthe light coming from the radiation source does not contribute to thelight produced by the illumination system;

FIG. 3 is a schematic representation of the emission wavelengths of atwo phorphor system, showing how the long wavelength end of one of thephosphors may be substantially the same as the short wavelength end ofthe other phosphor;

FIG. 4 is a schematic representation of the emission wavelengths of atwo phorphor system, wherein in this embodiment there is a wavelengthgap between the long wavelength end of one of the phosphors and theshort wavelength end of the other phosphor;

FIG. 5 is an emission spectra of an exemplary phosphate-based yellowphosphor according to the present embodiments, showing how a variationin the amount of phosphorus in the phosphor affects emission wavelengthslightly, and the emission intensity strongly;

FIG. 6 shows a comparison of yellow phosphors having either Ba and Sr,or Ba and Ca, or Sr alone; specifically, the emission spectra of theyellow phosphors (Sr_(0.7)Ba_(0.3)Eu_(0.02))₂Si_(1.02)O_(4.08),(Ca_(0.5)Ba_(0.5)Eu_(0.02))₂Si_(1.02) O_(4.08), and(Sr_(0.7)Eu_(0.02))₂Si_(1.02)O_(4.08), tested under 400 nm excitationradiation, are shown;

In FIG. 7 is an emission spectrum of an exemplary yellow phosphorcontaining all three of the elements Ba, Sr, and Ca; the specificphosphor tested in FIG. 7 is(Ba_(0.5)Sr_(0.5-x)Ca_(x)Eu_(0.02))₂Si_(1.02)O_(4.08), where the valueof x has been varied between 0.15 and 0.35;

FIG. 8 is an emission spectra of the exemplary yellow phosphors(Sr_(0.7)Ba_(0.3)Eu_(0.02))₂Si_(1.02)O_(4.08) and(Sr_(0.7)Ba_(0.3)Eu_(0.02))₂Si_(1.02)O_(4.08-x)F_(x) tested under 400 nmexcitation radiation, showing that the emission intensity issignificantly increased by doping the composition with fluorine;

FIGS. 9A and 9B are excitation and emission spectra of the exemplaryyellow phosphor (Sr_(0.7)Ba_(0.3)Eu_(0.02))₂Si_(1.02)O_(4.08-x)F_(x)tested in comparison with a commercial YAG yellow phosphor;

FIG. 10 describes an exemplary silicate-based blue phosphor where thephosphor may have the formula (Sr_(x)Mg_(1-x)Eu_(0.02))₂SiO₄ and wherethe Eu is present in small amounts as a luminescent activator; the Srmay be used to replace the Mg in a single phase solid solution up toabout x=0.5 concentration because two separate phases Mg₂SiO₄ andSr₂SiO₄ will be formed when the Sr substitution x is 0.6 (as shown incurve 3 of FIG. 10);

FIG. 11 is an emission spectrum of an exemplary aluminate-based bluephosphor, the phosphor having the formula Sr_(1-x)MgEu_(x)Al₁₀O₁₇ where0.2≦x≦1.0, and wherein a higher concentration of the Eu enhances thepeak luminescent intensity and shifts the peak wavelength to longerwavelengths;

FIG. 12 is an emission spectrum of an alternative exemplaryaluminate-based blue phosphor described by the formula(Sr_(3.9)Eu_(0.1))Al₁₄O₂₅;

FIG. 13 is a schematic representation of the emission wavelength from asingle phosphor system used in conjunction with a non-visible (topurple) radiation source;

FIG. 14 shows the emission spectra of a white LED having a two phosphorsystem, wherein the two phosphors of the phosphor system are blue andyellow with one being a phosphate-based yellow phosphor and the otherbeing an aluminate based blue phosphor; the two phosphors are mixed indifferent ratios such that a high blue phosphor ratio is desired whennon-visible UV LED is used for excitation;

FIG. 15 shows the emission spectra of a white LED constructed from fivedifferent phosphor systems designed for near-UV LED; two are blue andyellow two-phosphor systems, one is a two phosphor system having twoyellow phosphors, and two are three-phosphor systems having two yellowand one blue phosphors; and

FIG. 16 is a calculation of the color temperature of the three phosphorsystems described in FIG. 14.

DETAILED DESCRIPTION OF THE INVENTION

The white light illumination system of the present embodiments dependson an excitation source that does not contribute substantially to thewhite light output of the system because the exication source emits in aregion of the electromagnetic spectrum that is not visible to the humaneye. The concepts are illustrated schematically in FIGS. 1 and 2.

Referring to the prior art system 10 of FIG. 1, a radiation source 11(which may be an LED) emits light 12, 15 in the visible portion of theelectromagnetic spectrum. Light 12 and 15 is the same light, but isshown as two separate beams for illustrative purposes. A portion of thelight emitted from radiation source 11, light 12, excites a phosphor 13,which is a photoluminescent material that is capable of emitting light14 after absorbing energy from the LED 11. The light 14 is typicallyyellow. Radiation source 11 also emits blue light in the visible that isnot absorbed by the phosphor 13; this is the visible blue light 15 shownin FIG. 1. The visible blue light 15 mixes with the yellow light 14 toprovide the desired white illumination 16 shown in the figure. Adisadvantage of the prior art illumination system 10 of FIG. 1 is thatthe color output of the system 10 depends on the output 15 of theradiation source 11.

The color output of the present white light illumination system does notvary significantly with the color output of the radiation source (e.g.,LED) if the white light emitted by the system does not emit radiation ata wavelength that is significantly visible to the human eye. Forexample, and LED may be constructed to emit ultraviolet (UV) radiationhaving a wavelength of 380 nm or less, which is not visible to the humaneye. Furthermore, the human eye is not very sensitive to UV radiationhaving a wavelength between about 380 and 400 nm, nor is itsubstantially sensitive to violet light having a wavelength betweenabout 400 and 420 nm. Therefore, the radiation emitted by a sourcehaving a wavelength of about 420 nm or less will not substantiallyaffect the color output of the white light illumination system.

This aspect of the present invention is illustrated in FIG. 2. Referringto FIG. 2, substantially non-visible light is emitted from radiationsource 21 as light 22, 23. Light 22, 23 has the same characteristics,but different reference numerals have been used to illustrate thefollowing point: light 22 may be used to excite a phosphor, such asphosphor 24 or 25, but that light 23 emitted from the radiation source21 which does not impinge on a phosphor does not contribute to the coloroutput 28 from the phosphor(s) because light 23 is substantiallyinvisible to the human eye. In one embodiment of the present invention,radiation source 21 is an LED that emits light having a wavelengthgenerally ranging from about 250 to 410 nm. In alternative embodiments,radiation sources having excitation wavelengths up to 420 nm arefeasible. It will be understood by those skilled in the art that near UVradiation 400 nm and higher may contribute to the color rendition of thewhite light emitted from the white light LED if the radiation source isstrong enough in its intensity.

A second way to avoid affecting the color output of the whiteillumination system 30 is to configure the luminescent materials 24, 25(referring to FIG. 2) such that they each have a thickness that issufficient to prevent radiation from the LED 21 from passing through thematerial. For example, if the LED emits visible light between about 420and 650 nm, then in order to ensure that the phosphor thickness does notaffect the color output of the system, the phosphor should be thickenough to prevent any significant amount of the visible radiationemitted by the LED from penetrating through the phosphor.

Prior art methods have included phosphors that may be excited byultraviolet LED's. U.S. Pat. No. 6,555,958 to Srivastava et al.described a blue-green illumination system excited by a UV LED emittingin the range 360-420 nm. The luminescent material was a phosphor havingthe material composition Ba₂SiO₄:Eu⁺², Ba₂(MgZn)Si₂O₇:Eu⁺², and/orBa₂Al₂O₄:Eu⁺². Although this patent teaches the use of a non-visibleradiation source, there is only one phosphor designed to produce lightin the blue-green region of the spectrum.

A prior art method designed to emit white light using a non-visible UVLED was disclosed in U.S. Pat. No. 6,621,211 to Srivastava et al. Thewhite light produced in this method was created by non-visible radiationimpinging on three, and optionally a fourth, phosphor, of the followingtypes: the first phosphor emitted orange light having a peak emissionwavelength between 575 and 620 nm, and preferably comprised a europiumand manganese doped alkaline earth pyrophosphate phosphor according tothe formula A₂P₂O₇:Eu²⁺, Mn²⁺. Alternatively, the formula for the orangephosphor could be written (A_(1-x-y)Eu_(x)Mn_(y))₂P₂O₇, where 0<x≦0.2,and 0<y≦0.2. The second phosphor emits blue-green light having a peakemission wavelength between 495 and 550 nm, and is a divalent europiumactivated alkaline earth silicate phosphor ASiO:Eu²⁺, where A comprisedat least one of Ba, Ca, Sr, or Mb. The third phosphor emitted blue lighthaving a peak emission wavelength between 420 and 480 nm, and comprisedeither of the two commercially available phosphors “SECA,”D₅(PO₄)₃Cl:Eu²⁺, where D was at least one of Sr, Ba, Ca, or Mg, or“BAM,” which may be written as AMg₂Al₁₆O₂₇, where A comprised at leastone of Ba, Ca, or Sr, or BaMgAl₁₀O₁₇:Eu²⁺. The optional fourth phosphoremits red light having a peak emission wavelength between 620 and 670nm, and it may comprise a magnesium fluorogermanate phosphorMgO*MgF*GeO:Mn⁴⁺.

The prior art described above made use of a non-visible radiation sourceto excite the phosphors of the illumination system, but not described inthat art was a white light illumination system that utilized anon-visible radiation source with relatively few phosphors. U.S. Pat.No. 6,555,958 disclosed only one phosphor, but the application was for ablue-green illumination system, and so only one phosphor was needed. Theapplication in U.S. Pat. No. 6,621,211 was for a white lightillumination system, but three (and optionally a fourth) phosphors wererequired to generate white light. What has not been described in theprior art is a white light illumination system that uses only one, two,and optionally a third phosphor with a non-visible radiation system. Theadvantages of reducing the number of phosphors include ease ofmanufacture, cost, and quality of the white light thus produced. Thenext section will focus on the novel phosphors of the presentembodiments.

Emission Wavelength Ranges of the Phosphors

According to embodiments of the present invention, a white lightillumination system is provided comprising two phosphors. In thisembodiment, a first phosphor is configured to absorb at least a portionof radiation emitted from a non-visible radiation source and emit lightin one wavelength range, and a second phosphor configured to absorb atleast a portion of radiation emitted from a non-visible radiation sourceand emit light in a second wavelength range, where the longestwavelength of one of the ranges is substantially the same as theshortest wavelength of the other. This concept is illustrated in FIG. 3.Referring to FIG. 3, a phosphor system shown generally at 30 comprises afirst phosphor 31 having a peak emission within the wavelength rangedepicted by the reference numeral 32, and a second phosphor 33 having apeak emission within the range depicted by the reference numeral 34. Theshort wavelength end of phosphor 31 is represented by the referencenumeral 35, and the long wavelength end of phosphor 31 represented bywavelength 36. Similarly, the long wavelength end of phosphor 33 isrepresented by wavelength 37, and the short wavelength end of phosphor33 is represented by numeral 36. Hence, the long wavelength end ofphosphor 31 is substantially the same as the short wavelength end ofphosphor 33.

In an example of this embodiment, the phosphor 33 may be a yellowphosphor configured to absorb at least a portion of the radiation fromthe radiation source and emit light in a wavelength ranging from about530 to 590 nm, and the phosphor 31 may be a blue phosphor configured toabsorb at least a portion of the radiation from the radiation source andemit light in a wavelength ranging from about 470 to 530 nm. In thisembodiment, the longest wavelengths of the blue phosphor's emission aresubstantially equal to the shortest wavelengths of the yellow phosphor'semission. The radiation source in this embodiment may be an LED designedto emit radiation in the non-visible portion of the electromagneticspectrum, or in a portion of the spectrum where the human eye is notsubstantially sensitive, such as an LED emitting in a wavelength rangingfrom about 250 to 420 nm;

In an alternative embodiment, a two phosphor system may be designedsimilar to the system of FIG. 3, except where there is a slight gapbetween the longest wavelength of the first phosphor, and the shortestwavelength of the second phosphor. This concept is illustrated in FIG.4. Referring to FIG. 4, a phosphor system shown generally at 40comprises a first phosphor 41 having a peak emission within thewavelength range depicted by the reference numeral 42, and a secondphosphor 43 having a peak emission within the range depicted by thereference numeral 44. The short wavelength end of phosphor 41 isrepresented by the reference numeral 45, and the long wavelength end ofphosphor 41 represented by wavelength 46A. Similarly, the longwavelength end of phosphor 43 is represented by wavelength 47, and theshort wavelength end of phosphor 43 is represented by numeral 46B.Hence, a gap 48 is developed between the long wavelength end of phosphor41 and the short wavelength end of phosphor 43. Adjusting the size ofthe wavelength gap 48 to enhance the quality of the white light providedby the white light illumination system is one of the novel features ofthe present embodiments.

By way of example, the phosphor 43 may be a yellow phosphor configuredto absorb at least a portion of the radiation from the radiation sourceand emit light in a wavelength ranging from about 540 to 580 nm, and thephosphor 41 may be a blue phosphor configured to absorb at least aportion of the radiation from the radiation source and emit light in awavelength ranging from about 480 to 510 nm. Thus, the gap 48 in thisembodiment would be about 30 nm. In an alternative embodiment, thephosphor 43 may be a yellow phosphor configured to emit light in awavelength ranging from about 550 to 575 nm, and the phosphor 41 may bea blue phosphor configured to emit light in a wavelength ranging fromabout 480 to 495 nm. In this embodiment, the gap 48 would be about 55nm.

Exemplary Yellow Phosphors

According to embodiments of the present invention, the yellow phosphor33, 43 may be either a silicate-based phosphor or a phosphate-basedphosphor. Additionally, the yellow phosphor 33, 43 may have the formulaM₁M₂M₃SiO₄, wherein M₁, M₂ and M₃ are individually either Sr, Ca, or Ba.If the yellow phosphor 33, 43 is a silicate-based phosphor, then it mayhave the formula Sr_(1-x-y)Ba_(x)Ca_(y)SiO₄; where 0≦x≦0.8, and 0≦y≦0.8.In alternative embodiments, the silicate-based, yellow phosphor 33, 34may conform to the same formula where 0≦x≦0.5 and 0≦y≦0.3, or where0.5≦x≦0.7 and 0.2≦y≦0.5. Alternatively, the yellow phosphor 33, 34 maybe a phosphate-based phosphor according to the formula(Sr_(1-x-y)Eu_(x)Mn_(y))₂P_(2+z)O₇, where 0.03≦x≦0.08, 0.06≦y≦0.16, and0<z≦0.05.

As a specific example, the phosphate-based yellow phosphor having theformula (Sr_(0.8)Mn_(0.16)Eu_(0.4))₂P_(2+x)O₇ is now discussed inrelation to FIG. 5, which is an emission spectra showing how a variationin the amount of phosphorus in the phosphor affects emission wavelengthslightly, and the emission intensity strongly. The phosphor was excitedby a radiation source having a wavelength of about 400 nm. FIG. 5 showsthat increasing the phosphorus stoichiometry of this phosphor from 2.00to 2.02 increases the intensity of the emission, but then furtherincreasing the stoichiometry to 2.06 decreases the intensity of theemission.

Examples of silicate-based yellow phosphors 33, 43 are shown in FIGS.6-9. FIG. 6 shows a comparison of yellow phosphors having either Ba andSr, or Ba and Ca, or Sr alone. Specifically, the emission spectra of theyellow phosphors (Sr_(0.7)Ba_(0.3)Eu_(0.02))₂Si_(1.02)O_(4.08),(Ca_(0.5)Ba_(0.5)Eu_(0.02))₂Si_(1.02)O_(4.08), and(Sr_(0.7)Eu_(0.02))₂Si_(1.02)O_(4.08), tested under 400 nm excitationradiation, are shown in FIG. 6. The data shows that the emissionspectrum may be adjusted from green-yellow (with an emission peakoccurring at about 520 nm) to orange-yellow (with an emission peakoccurring at roughly 580 nm) by varying the content of the Sr, Ba,and/or Ca in this silicate-based phosphor system. By decreasing theamount of Ba slightly and replacing Ca with Sr, the peak emissionwavelength may be increased toward yellow. By further increasing theamount of the Sr and eliminating the Ba content, the peak emissionwavelength may be increased further.

In FIG. 7, a yellow phosphor according to the present embodiments isdescribed wherein the phosphor in this example contains all three of theelements Ba, Sr, and Ca. The specific phosphor tested in FIG. 7 is(Ba_(0.5)Sr_(0.5-x)Ca_(x)Eu_(0.02))₂Si_(1.02)O_(4.08), where the valueof x has been varied between 0.15 and 0.35.

In FIG. 8, the emission spectra of the exemplary yellow phosphors(Sr_(0.7)Ba_(0.3)Eu_(0.02))₂Si_(1.02)O_(4.08) and(Sr_(0.7)Ba_(0.3)Eu_(0.02))₂Si_(1.02)O_(4.08-x)F_(x) were tested under400 nm excitation radiation. Here, the data shows that the emissionintensity is significantly increased by doping the composition withfluorine, or by substituting some of the oxygen content of thecomposition with fluorine, while the wavelength of the spectrummaintains substantially unchanged.

Additional advantages of the exemplary yellow phosphors of the presentembodiments are demonstrated by the excitation and emission spectrashown in FIGS. 9A and 9B. FIG. 9B is an emission spectra of the yellowphosphor (Sr_(0.7)Ba_(0.3)Eu_(0.02))₂Si_(1.02)O_(4.08):F_(x), where x(which denotes the F content) ranges from 0 to 0.05. The conditionsunder which this fluorine containing material were tested include 450 nmexcitation radiation in order to compare the test results with theemission of a commercial YAG yellow phosphor previously reported in theliterature. Furthermore, this excitation wavelength of 450 nm was chosento compare emission intensities because this is the wavelength where thecommercial YAG phosphor is the most responsive, as shown in FIG. 9A.FIG. 9A shows that the YAG phosphor may only be only excited byradiation having a peak wavelength around either 340 nm or 470 nm,whereas the present Sr_(0.7)Ba_(0.3)Eu_(0.02))₂Si_(1.02)O_(4.08-x)F_(x)phosphor may be excited by a much broader range of wavelengths, rangingfrom the entire UV to the blue region of the spectrum (i.e., 280 nm to470 nm).

Exemplary Blue Phosphors

According to embodiments of the present invention, the blue phosphor 31,41 may be either a silicate-based phosphor or an aluminate-basedphosphor. An exemplary silicate-based blue phosphor has the formulaSr_(1-x-y)Mg_(x)Ba_(y)SiO₄; where 0≦x≦1.0, and 0≦y≦1.0. In anotherembodiment, 0.2≦x≦1.0 and 0≦y≦0.2. In this embodiment, Mg and Ba areused to replace Sr in the composition.

In an alternative embodiment of the silicate-based blue phosphor, thephosphor may have the formula (Sr_(x)Mg_(1-x)Eu_(0.02))₂Si_(1.02)O₄,where Eu is present in small amounts as an activator; Sr may be used toreplace Mg in increasing amounts in the material; and the amount of Siin a SiO₄ host is present in an amount greater than a stoichiometricratio of 1 to 4. Experimental results for such a phosphor are shown inFIG. 10. Referring to FIG. 10, the stoichiometric amount of Sr in thecomposition has been increased from 20 percent (curve 1) to 40 percent(curve 2), and then increased further to 60 percent (curve 3). Thewavelength of the excitation radiation was 400 nm. The results show thatblue emission can be efficiently excited by UV to blue radiation whenthe strontium substitution for magnesium is less than 40 percent in thissilicate-based system. Two phases of silicates (Mg₂SiO₄ and Sr₂SiO₄) areformed when the strontium substitution is higher than 40 percent, andthen at least part of the emission shifts to substantially longerwavelengths.

Alternatively, the blue phosphor in the present embodiments may bealuminate-based. In one embodiment of an aluminate-based blue phosphor,the phosphor has the formula Sr_(1-x)MgEu_(x)Al₁₀O₁₇ where 0.2<x≦1.0.This novel blue phosphor utilizes Eu in a substantially higher amountthan that amunt used in the prior art. The emission spectra of such aphosphor is shown in FIG. 11 where “x,” the Eu content, has beenmeasured at 20, 40, 60, and 80 stoichiometric percent. In thesecompositions, the Eu is substituting for the Sr, and the wavelength ofthe excitation radiation was 400 nm. The results show that the Euconcentration affects both the intensity and the wavelength of theemission from this blue phosphor: increasing the Eu content increasesboth the intensity and the wavelength.

In other embodiments of an aluminate-based blue phosphor, the phosphorhas the formula Sr_(3.9) Eu_(0.1)Al₁₄O₂₅. Experimental results for thisphosphor are illustrated in FIG. 12, where an emission spectrum for theexemplary aluminate-based blue phosphor (Sr_(3.9)Eu_(0.1))Al₁₄O₂₅ wasmeasured using 400 nm excitation radiation. The data shows that the peakof the emission is about 500 nm, and the emission ranges from about 460to 480 nm. Another way to describe this blue aluminate phosphor is bythe formula Sr_(x)Eu_(0.1)Al₁₄O₂₅, wherein x is less than 4. In otherwords, the content of the Sr in the composition is less than 4, and itmay be 3.9.

Relative Amounts of the Phosphors and Emission Spectra of the White LEDs

It will be understood by those skilled in the art that the present novelphosphor systems may be used in a variety of configurations, and it iscertainly not necessary (although it may be desired in some situations)to use one yellow and one blue phosphor. The flexibility of this systemis demonstrated in this section by giving exemplary emission spectra ofthe white LED for several different phosphor systems. In FIG. 14, forexample, white light may be generated by using a non-visible UV LED toexcite the blue and yellow phosphors according to the presentembodiments, and the intensity of the resultant white light may beadjusted by varying the ratio of the two phosphors present in thephosphor system.

Referring to FIG. 14, a white light LED was constructed by mixing aphosphate-based yellow phosphor with an aluminate based blue phosphor,and exciting the two-phosphor mixture with a radiation source thatprovided excitation radiation ranging from about 370 to 400 nm (in otherwords, non-visible radiation). The phosphate-based yellow phosphor was(Sr_(0.8)Mn_(0.16)Eu_(0.04))₂P_(2.02)O₇, and the aluminate-base bluephosphor was Sr_(0.2)MgEu_(0.8)Al₁₀O₁₇. Three-phosphor mixtures werealso tested, wherein the relative amounts of the aluminate-based bluephosphor to the phosphate-based yellow phosphor was about 50 to 50weight percent in curve “A,” 40 to 60 weight percent in curve “B,” and30 to 70 weight percent in curve “C.” The data shows that the overallemission spectra can be tailored to achieve the desired effect byadjusting the ratio of the amounts of blue to yellow phosphors, thusoptimizing the color rendering for different applications as shown inFIG. 16.

A second example of the effect of varying phosphor ratios in two andthree phosphor systems is shown in FIG. 15. Here, phosphor A is thephosphate-based yellow phosphor (Sr_(0.8)Mn_(0.16)Eu_(0.04))₂P_(2.02)O₇;phosphor B is the silicate-based yellow phosphor(Ba_(0.3)Sr_(0.7)Eu_(0.02))₂Si_(1.02)O_(4.08); phosphor C is thealuminate-based blue phosphor Sr_(0.2)MgEu_(0.8)Al₁₀O₁₇; and phosphor Dis the aluminate-based blue phosphor Sr_(3.9)Eu_(0.1)Al₁₄O₂₅. Curve 1 isthe emission spectra from a two phosphor system of 84 weight percentphosphor B and 16 weight percent phosphor D; curve 2 is from a twophosphor system having 83 weight percent phosphor B and 17 weightpercent phosphor C; curve 3 is from a two phosphor system of 36 weightpercent A and 64 weight percent B; curve 4 is from a three phosphorsystem having 46 weight percent A, 42 weight percent B, and 12 weightpercent D, and curve 5 is from a three phosphor system having 61 weightpercent A, 19 weight percent B, and 20 weight percent C.

The data for the exemplary two and three-phosphor systems in FIG. 15shows that the greatest intensity emissions and the shortest wavelengthemission peaks occurs for the blue and yellow two-phosphor systems(curves 1 and 2). Intermediate is the two-phosphor system consisting ofa blue and yellow phosphor (curve 3). Although the intensity of thethree-phosphor systems consisting of two yellow and one blue phosphors(curves 4 and 5) are the lowest of the group, these emissions have thelongest wavelength peak emissions, being shifted more towards the redregion of the spectrum.

A calculation of the color temperature of the white light from the threeratios of the two yellow phosphor system of FIG. 14 is shown in FIG. 16.

Single Phosphor Systems

In other embodiments of the present invention, a single phosphor may beused with a non-visible (to purple) radiation source. This concept isillustrated in FIG. 13 in the schematic shown generally at 130, where aphosphor 131 has a short wavelength end 135, and a long wavelength end136. The single phosphor is used in conjunction with a radiation sourceproviding excitation radiation to the single phosphor in the non-visibleto purple region of the spectrum, the excitation radiation ranging fromabout 250 to 430 nm.

An exemplary phosphor for a single phosphor system is the silicate-basedyellow phosphor containing all three of the elements Ba, Sr, and Ca, andcontaining a small amount of an Eu activator. An example of such aphosphor has already been shown in FIG. 7, where the specific phosphor(Ba_(0.5)Sr_(0.5-x)Ca_(x)Eu_(0.02))₂Si_(1.02)O_(4.08), was tested forvalues of the Ca content (“x”) varying between 0.15 and 0.35. In thisembodiment, it is advantageous to utilize an excitation radiation havinga wavelength range of about 400 to 430 nm from the non-visible to purpleLED. This exemplary phosphor was tested to have an emission wavelengthpeak at about 540 nm, a short wavelength end 135 (in reference to FIG.13) of about 480 nm, and a long wavelength end 136 (again, in referenceto FIG. 13) of about 640 nm.

In an alternative embodiment of the single phosphor concept, the singlephosphor may be a silicate-based yellow phosphor having a small amountof Eu and a composition containing only Sr, such as the phosphorrepresented by curve 3 in FIG. 6. Here the emission wavelength peakoccurs at about 570 nm; the short wavelength end of the emission occursat about 500 nm, and the long wavelength end occurs at about 680 nm.

Thus, it is demonstrated that it is feasible to construct a white lightillumination system using a non-visible to purple LED as a radiationsource, and a single phosphor receiving excitation radiation from theradiation source and emitting white light illumination.

Methods of Preparing the Phosphor System

Exemplary methods of preparing the present phosphors include a sol-gelmethod and a solid reaction method. The sol-gel method may be used toproduce powder phosphors, including those that are aluminate, phosphateand silicate-based. A typical procedure comprised the steps of:

-   -   1. a) Dissolving certain amounts of alkaline earth nitrates (Mg,        Ca, Sr, Ba), and Eu₂O₃ and/or Mn(NO₃)₂ in dilute nitric acid;        and        -   b) Dissolving corresponding amount of aluminum nitrate,            NH₄H₂PO₄ or silica gel w in de-ionized water to prepare a            second solution.    -   2. After the solids of the two solutions of steps 1 a) and 1 b)        above were substantally in solution, the two solutions were        mixed and stirred for two hours. Ammonia was then used to        generate a gel in the mixture solution. Following formation of        the gel, the pH was adjusted to about 9.0, and the gelled        solution stirred continuously stirred at about 60° C. for about        3 hours.    -   3. After drying the gelled solution by evaporation, the resulted        dry gel was calcined at 500 to 700° C. for about 60 minutes to        decompose and acquire oxides.    -   4. After cooling and grinding, the solid was sintered in a        reduced atmosphere for about 6 to 10 hours. For aluminate and        silicate-based phosphors, flux was used to improve the sintering        properties, and the sintering temperature ranged from about 1300        to 1500° C. For the phosphate-based phosphors, the sintering        temperature ranged from about from 900 to 1100° C.    -   5. For fluorine- containing silicate phosphors, NH₄F powder with        about 5 weight percent was used to mix with the cacined        stoichiometric silicates before the final sintering in reduction        atomosphere. The sintering temperature was usually about 100° C.        lower than the non-fluorine containing silicates since fluorides        may under certain conditions act as flux agents.

Alternatively, the solid reaction method was used to produce the powderphosphors including aluminate, phosphate and silicate-based phosphors.The steps of a typical procedure used for the solid reaction method areas follows:

-   -   1. Desired amounts of alkaline earth oxides or carbonates (Mg,        Ca, Sr, Ba), dopants of Eu₂O₃ and/or MnO, corresponding Al₂O₃,        NH₄H₂PO₄ or SiO₂ were wet mixed with a ball mill.    -   2. After drying and grinding, the resulting powder was sintered        in a reduced atmosphere for about 6 to 10 hours. For the        aluminate and silicate-based phosphors, flux was used to improve        the sintering properties, and the sintering temperature ranged        from 1300 to 1500° C. For the phosphate-based phosphors, the        sintering temperature ranged from about 900 to 1100° C.

Many modifications of the exemplary embodiments of the inventiondisclosed above will readily occur to those skilled in the art.Accordingly, the invention is to be construed as including all structureand methods that fall within the scope of the appended claims.

1. A white LED comprising: a radiation source configured to emitradiation having a wavelength ranging from about 250 to 420 nm; a yellowphosphor configured to absorb at least a portion of the radiation fromthe radiation source and emit light with peak intensity in a wavelengthranging from about 530 to 590 nm; and a blue phosphor configured toabsorb at least a portion of the radiation from the radiation source andemit light with peak intensity in a wavelength ranging from about 470 to530 nm.
 2. The white LED of claim 1, wherein a 250 to 400 nm portion ofthe radiation emitted by the radiation source is substantiallynon-visible ultraviolet (UV) radiation, and a 400 to 420 nm portion isnear-UV.
 3. The white LED of claim 1, wherein the radiation sourcecomprises a light emitting diode (LED).
 4. The white LED of claim 3,wherein the radiation source comprises at least one semiconductor layerselected from the group consisting of GaN, ZnSe, and SiC, and at leastone active region comprising ap-n junction selected from the groupconsisting of GaN, AlGaN, InGaN, and InAlGaN.
 5. The white LED of claim1, wherein the yellow phosphor is selected from the group consisting ofsilicate-based phosphors and phosphate-based phosphors.
 6. The white LEDof claim 5, wherein the yellow phosphor has the formula A₂SiO₄:Eu²⁺F,and wherein A is at least one of a divalent metal selected from thegroup consisting of Sr, Ca, Ba, Mg, Zn, and Cd.
 7. The white LED ofclaim 6, wherein the yellow phosphor has the formulaSr_(1-x-y)Ba_(x)Ca_(y)SiO₄:Eu⁺²F; and where 0≦x≦0.8; and 0≦y≦0.8.
 8. Thewhite LED of claim 6, wherein the yellow phosphor has the formulaSr_(1-x-y)Ba_(x)Ca_(y)SiO₄:Eu²⁺F; and where 0≦x≦0.5; and 0≦y≦0.3.
 9. Thewhite LED of claim 6, wherein the yellow phosphor has the formulaSr_(1-x-y)Ba_(x)Ca_(y)SiO₄: Eu²⁺F; and where 0.5≦x≦0.7; and 0.2≦y≦0.5.10. The white LED of claim 5, wherein the yellow phosphor has theformula (Sr_(1-x-y)Eu_(x)Mn_(y))₂P_(2+z)O₇; and where 0.03≦x≦0.08;0.06≦y≦0.16; and 0<z≦0.05.
 11. The white LED of claim 1, wherein theblue phosphor is selected from the group consisting of silicate-basedphosphors and aluminate-based phosphors.
 12. The white LED of claim 11,wherein the blue phosphor has the formulaSr_(1-x-y)Mg_(x)Ba_(y)SiO₄:Eu²⁺F; and where 0.5≦x≦1.0; and 0≦y≦0.5. 13.The white LED of claim 11, wherein the blue phosphor has the formulaSr_(1-x)MgEu_(x)Al₁₀O₁₇; and where 0.01<x≦1.0.
 14. The white LED ofclaim 11, wherein the blue phosphor has the formula Sr_(x)Al₁₄O₂₅:Eu⁺²,and where x<4.
 15. A white LED comprising: a radiation source configuredto emit radiation having a wavelength ranging from about 250 to 420 nm;a yellow phosphor configured to absorb at least a portion of theradiation from the radiation source and emit light with a peak intensityin a wavelength ranging from about 550 to 590 run; and a blue phosphorconfigured to absorb at least a portion of the radiation from theradiation source and emit light with a peak intensity in a wavelengthranging from about 480 to 510 nm.
 16. The white LED of claim 15, whereina 250 to 400 nm portion of the radiation emitted by the radiation sourceis substantially non-visible ultraviolet (UV) radiation, and a 400 to420 nm portion is near-UV.
 17. The white LED of claim 15, wherein theradiation source comprises a light emitting diode (LED).
 18. The whiteLED of claim 17, wherein the radiation source comprises at least onesemiconductor layer selected from the group consisting of GaN, ZnSe, andSiC, and at least one active region comprising ap-n junction selectedfrom the group consisting of GaN, AlGaN, InGaN, and InAlGaN.
 19. Thewhite LED of claim 15, wherein the yellow phosphor is selected from thegroup consisting of silicate-based phosphors and phosphate-basedphosphors.
 20. The white LED of claim 19, wherein the yellow phosphorhas the formula A₂SiO₄:Eu²⁺F, and wherein A is at least one of adivalent metal selected from the group consisting of Sr, Ca, Ba, Mg, Zn,and Cd.
 21. The white LED of claim 20, wherein the yellow phosphor hasthe formula Sr_(1-x-y)Ba_(x)Ca_(y)SiO₄:Eu⁺²F; and where 0≦x≦0.8; and0≦y≦0.8.
 22. The white LED of claim 20, wherein the yellow phosphor hasthe formula Sr_(1-x-y)Ba_(x)Ca_(y)SiO₄:Eu²⁺F; and where 0≦x≦0.5; and0≦y≦0.3.
 23. The white LED of claim 20, wherein the yellow phosphor hasthe formula Sr_(1-x-y)Ba_(x)Ca_(y)SiO₄: Eu²⁺F; and where 0.5≦x≦0.7; and0.2≦y≦0.5.
 24. The white LED of claim 19, wherein the yellow phosphorhas the formula (Sr_(1-x-y)Eu_(x)Mn_(y))₂P_(2+z)O₇; and where0.03≦x≦0.08; 0.06≦y≦0.16; and 0<z≦0.05.
 25. The white LED of claim 15,wherein the blue phosphor is selected from the group consisting ofsilicate-based phosphors and aluminate-based phosphors.
 26. The whiteLED claim 25, wherein the blue phosphor has the formulaSr_(1-x-y)Mg_(x)Ba_(y)SiO₄:Eu₂₊F; and where 0.5≦x≦1.0; and 0≦y≦0.5. 27.The white LED of claim 25, wherein the blue phosphor has the formulaSr_(1-x)MgEu_(x)Al₁₀O₁₇; and where 0.01<x≦1.0.
 28. The white LED ofclaim 25, wherein the blue phosphor has the formula Sr_(x)Al₁₄O₂₅:Eu⁺²,and where x≦4.
 29. A single phosphor system for a white LED, thephosphor system comprising: a radiation source configured to emitradiation having a wavelength ranging from about 250 to 420 nm; aphosphor configured to absorb at least a portion of the radiation fromthe radiation source and emit a broad spectrum light with a peakintensity in a wavelength ranging from about 520 to 560 nm.
 30. Thewhite LED of claim 29, wherein the 250 to 400 nm portion of theratiation emitted by the radiation source is substantially non-visibleultraviolet (UV) radiation, and the 400 to 420 nm portion issubstantially purple light from the visible range of the electromagneticspectrum.
 31. The white LED of claim 29, wherein the radiation sourcecomprises a light emitting diode (LED).
 32. The white LED of claim 31,wherein the radiation source comprises at least one semiconductor layerselected from the group consisting of GaN, ZnSe, and SiC, and at leastone active region comprising a p-n junction selected from the groupconsisting of GaN, AlGaN, InGaN, and InAlGaN.
 33. The white LED of claim29, wherein the single phosphor is selected from the group consisting ofsilicate-based phosphors.
 34. The white LED of claim 29, wherein thesingle phosphor has the formula A₂SiO₄:Eu⁺²F, wherein A is at least oneof a divalent metal selected from the group consisting of Sr, Ca, Ba,Mg, Zn, and Cd.
 35. The white LED of claim 29, wherein the singlephosphor has the formula Sr_(1-x-y)Ba_(x)Ca_(y)SiO₄:Eu⁺²F; and where0.3≦x≦0.8; and 0.1≦y≦0.5.
 36. The white light illumination system ofclaim 1, wherein the ratio of the yellow phosphor to the blue phosphorranges from about 9 to
 1. 37. The white light illumination system ofclaim 15, wherein the ratio of the yellow phosphor to the blue phosphorranges from about 9 to 0.2.
 38. White light illumination produced by thewhite LED of claim
 1. 39. White light illumination produced by the whiteLED of claim
 15. 40. White light illumination produced by the singlephosphor system of claim
 29. 41. The white light illumination accordingto claim 38, wherein the white light illumination comprises about 10 to50 percent light emitted by the blue phosphor and about 50 to 90 percentlight emitted by the yellow phosphor.
 42. The white light illuminationaccording to claim 39, wherein the white light illumination comprisesabout 20 to 50 percent light emitted by the blue phosphor and about 50to 80 percent light emitted by the yellow phosphor.
 43. The white lightillumination according to claim 1, wherein the white light illuminationhas a color temperature ranging from about 3000 to 6500 K.
 44. The whitelight illumination according to claim 15, wherein the white lightillumination has a color temperature ranging from about 3000 to 6500 K.45. The white light illumination according to claim 29, wherein thewhite light illumination has a color temperature ranging from about 3000to 6500 K.
 46. The white light illumination according to claim 1,wherein the white light illumination has a color rendering index (CRI)greater than about
 70. 47. The white light illumination according toclaim 15, wherein the white light illumination has a color renderingindex (CRI) greater than about
 70. 48. The white light illuminationaccording to claim 29, wherein the white light illumination has a colorrendering index (CRI) greater than about
 70. 49. A method of producingwhite light illumination from a two-phosphor system, the methodcomprising: providing a radiation source configured to emit radiationhaving a wavelength ranging from about 250 to 420 nm; exposing a yellowphosphor to at least a portion of the radiation from the radiationsource to produce light having a wavelength ranging from about 530 to590 nm; exposing a blue phosphor to at least a portion of the radiationfrom the radiation source to produce light having a wavelength rangingfrom about 470 to 530 nm; and mixing the light from the yellow phosphorwith the light from the blue phosphor to produce the white illumination.50. The white light illumination produced by the method of claim
 49. 51.The white light illumination of claim 50, wherein the white lightillumination comprises about 10 to 50 percent of the visible radiationfrom the blue region of the spectrum and about 50 to 90 percent of thevisible radiation from the yellow region of the spectrum.
 52. The whitelight illumination of claim 50, wherein the white light illumination hasa color temperature ranging from about 3000 to 6500 K.
 53. The whitelight illumination of claim 50, wherein the white light illumination hasa color rendering index (CRI) greater than about 70.